The Heat Number Is the Thread: Verifying EN 10204 3.1 Certificates for Chinese CNC Parts

The Certificate Is Not the Component

A mill certificate arriving as a PDF can create a comfortable impression: the material is stated, the chemistry is listed, the mechanical properties are within range, and the document carries a stamp. For a UK buyer sourcing CNC parts from China, that impression needs to be tested. EN 10204 3.1 is an inspection document, not a guarantee that the finished bracket, shaft or manifold in the crate was made from the heat it describes.

The distinction matters because CNC supply chains are physical chains before they are paper chains. A mill heat is cast, rolled or forged, sold through a stockholder, cut into bar or plate, issued to a machine shop, loaded onto a machine, and turned into a part. The certificate covers one end of that journey. The part carries the other. If the heat number does not connect them, the document proves very little about the component on the bench.

What a 3.1 Certificate Actually Certifies

Under EN 10204:2004, a 3.1 inspection certificate is issued by the manufacturer's authorised inspection representative who is independent of the manufacturing department. It states that the products supplied comply with the requirements of the order and gives the results of specific inspection and testing. In practice, for steel and aluminium mill products, that usually means heat analysis, mechanical properties, and sometimes hardness, impact or dimensional checks depending on the product standard and the purchase order.

It is not third-party certification. It is not a UKAS-accredited test report unless the testing laboratory happens to be accredited and the report says so. It is not a statement that every finished CNC part has been tested. Nor does it replace incoming goods inspection, first article approval or a supplier audit. The certificate is evidence about a heat of material; the buyer's task is to establish that the heat became the part.

Chinese suppliers often use the phrase 'mill test certificate' or 'material quality certificate'. Some are genuine 3.1 documents. Others are mill certificates issued to a different standard, stockholder certificates, or supplier-generated declarations with a mill logo attached. A UK purchase order should therefore specify EN 10204 3.1 explicitly, with the grade, product form and any additional testing requirements, rather than accepting an unspecified certificate as equivalent.

The Traceability Chain, Link by Link

A workable traceability chain has five links. First, the mill certificate identifies a heat number, the mill, the product standard, the grade, the delivery condition and the test results. Second, the stockholder or supplier records that heat number against the delivery note and the material certificate it passes to the machine shop. Third, the machine shop records the heat number on the cutting sheet, router or job card that accompanies the batch. Fourth, the heat number appears on the first article inspection report, material declaration or final inspection record for the finished part. Fifth, the packed parts or the batch label carry a reference that allows the buyer to retrieve those records.

Not every CNC job needs all five links on the part itself. Marking a heat number on a small aluminium component is often impractical. But the batch segregation must still allow the supplier to say which heat went into which delivery. If the supplier cannot move from a finished part or batch number back to a heat number and then to a 3.1 certificate, the paper chain is decorative rather than functional.

Where the Paper Chain Usually Breaks

The first break is the certificate issuer. A document headed with a trading company's name, carrying a mill's test results but no clear statement that it is issued under EN 10204 3.1 by the manufacturer's independent inspection representative, deserves scrutiny. The buyer should be able to identify the mill and the inspection function, not merely the supplier's quality department.

The second break is the heat number. Common failures include one certificate covering several heats without a clear allocation, a heat number that appears on the certificate but not on the delivery note, handwritten heat numbers added after the fact, and certificates reused across repeat deliveries. A heat number is only useful if it is exact and consistently transcribed. A single transposed digit can sever the chain.

The third break is the test result. Chemistry may be reported as product analysis rather than heat analysis, or mechanical properties may come from a test piece that is not representative of the delivered condition. A certificate can show values within specification while the grade, heat treatment or delivery condition on the certificate does not match the drawing requirement. The buyer should compare the certificate against the purchase order and the material specification, not just against a generic grade table.

The fourth break is the part. If the machine shop mixes offcuts from two heats, or if the cutting sheet records a stock item rather than a heat number, the finished batch may contain material from a heat that is not covered by the certificate presented. This is where a supplier's internal material control matters more than the mill's reputation.

A Verification Routine for UK Buyers

Start with the document. Check that it is addressed to the correct order or delivery, that the mill is identifiable, that the standard is EN 10204 3.1, and that the grade and product form match the drawing. Check the heat number character by character against the delivery note, the supplier's material record and the first article report. If the certificate is a translation, ask for the original-language version and compare the heat number and test values.

Next, verify the mill independently. Do not rely solely on contact details printed on the certificate. Use a separately found mill website or directory entry, and ask the mill's inspection department to confirm the certificate number and heat number. This is not always possible with large mills, but a supplier that cannot provide any route to the mill's own verification is relying on the buyer's trust rather than on traceability.

Then cross-check the technical content. Confirm that the chemistry ranges correspond to the specified grade, that the mechanical properties meet the drawing or product standard, and that the test methods and units are consistent. Watch for certificates that state a grade but list only typical values, or that report properties from a different delivery condition. For alloy verification, portable X-ray fluorescence can provide a useful incoming check on elements such as chromium, nickel and molybdenum, but it will not confirm carbon content or mechanical properties and should not be treated as a substitute for the certificate.

Finally, close the loop at goods-in. Record the heat number on the goods-in record, retain the certificate against the batch, and sample-check dimensions, hardness or marking as appropriate. If the supplier cannot link the delivery to a heat, quarantine the batch until the link is established.

When to Commission Independent Testing

Independent testing is not necessary for every consignment, but it is justified where the consequence of material failure is high. Examples include pressure-retaining parts, structural components, safety-critical assemblies, high-value batches, and any delivery where the certificate is incomplete, inconsistent or untraceable. It is also warranted where the grade is commonly counterfeited or where the supplier has changed mill without notice.

Commission testing against the drawing and product standard, not merely against the certificate. A UKAS-accredited laboratory in the UK or a CNAS-accredited laboratory in China can provide chemical analysis, tensile testing, hardness, impact testing or microstructural examination. The scope should be agreed in advance, and the samples should be traceable to the batch under dispute. Independent testing is most useful when it answers a defined question: is this heat the specified grade, and does it meet the required mechanical properties?

Putting It Into the Purchase Order

The most effective verification routine begins before the order is placed. The purchase order should require EN 10204 3.1 certificates for each heat, with the heat number stated on the delivery note and on the supplier's batch records. It should name the grade, product standard and delivery condition, and require the certificate to be issued by the material manufacturer. It should also state that certificates will be checked against the mill and that independent testing may be required at the buyer's discretion.

A certificate that says 3.1 is a starting point. The part says nothing unless the heat number, the batch record and the test results line up. For UK buyers sourcing CNC machined parts from China, the practical question is not whether the PDF looks official. It is whether the supplier can walk from the finished component back to a named heat at a named mill, and then show the inspection document that belongs to it.